DTV broadcasting in the United States of America has been done in accordance with broadcasting standards formulated by an industry consortium called the Advanced Television Systems Committee (ATSC), which standards have prescribed the use of a vestigial-sideband amplitude-modulated single carrier in each radio-frequency (RF) channel allocated for broadcasting DTV signals. Consideration is being given to replacing those DTV broadcasting standards with a new standard that prescribes coded orthogonal frequency-division multiplexed (COFDM) plural carriers in each RF channel allocated for broadcasting DTV signals. This new standard may, for example, resemble the DVB-T2 broadcasting standard developed for use in Europe.
COFDM is typically generated by data randomizing digital data to insure that subsequent encoding of forward-error-correction (FEC) coding receives sufficient density of logic ONEs to operate efficiently. Then, the resulting FEC coding is subjected to some form of bit interleaving, and the bits of the interleaved FEC coding are mapped to quadrature-amplitude-modulation (QAM) symbol constellations to implement a form of bit-interleaved coded modulation (BICM). The real-axis and imaginary-axis spatial coordinates of the QAM constellations are parsed into orthogonal frequency-division multiplex (OFDM) symbols, which modulate a single carrier wave at high rate using quadrature-amplitude-modulation (QAM). The resulting modulated carrier wave is then transformed in a fast inverse discrete Fourier transform (I-DFT) procedure to generate a multiplicity of baseband OFDM carriers. These baseband OFDM carriers are converted upward in frequency to generate radio-frequency (RF) carrier waves uniformly distributed within the frequency spectrum of the RF channel, each of which RF carriers is modulated at low symbol rate.
The DVB-T2 standard for DTV broadcasting prescribes Bose-Chaudhuri-Hocquenghem (BCH) coding concatenated with subsequent low-density parity-check (LPDC) coding as FEC coding. This coding is favored because of its relatively low redundancy as compared to turbo coding that provides similar performance in the presence of additive white Gaussian noise (AWGN). The concatenated BCH-LDPC coding prescribed in the DVB-T2 standard is reported to allow better performance in the presence of AWGN to be achieved using 256QAM constellations than could be achieved using 16QAM constellations per the earlier DVB-T standard for over-the-air DTV broadcasting. The bits of the LDPC coding are block interleaved using a column-twist or matrix type of interleaving in which successive bits of LDPC coding are arranged in columns for subsequent row-by-row utilization for mapping to lattice points within successive QAM constellations. The FEC frames of LDPC coding extend over very large numbers of bits, a normal FEC frame being composed of 64,800 bits and a short FEC frame used for transmissions to mobile receivers being composed of 16,200 bits.
The mapping of LDPC coding to square QAM constellations is Gray mapping, in which plural-bit code segments that are mapped by adjacent portions of the constellation differ in only a single bit. This reduces the bit errors caused by AWGN when de-mapping is done in the DTV receiver. However, soft de-mapping of the square QAM constellations in the DTV receiver recovers the successive bits of LDPC coding with varying respective levels of confidence that they are correct. Those bits of the de-mapping results with lower respective levels of confidence that they are correct are more likely adversely to affect the capability of LDPC decoding to recover the BCH-coded data as originally transmitted, than are those bits of the de-mapping results with higher respective levels of confidence that they are correct. The ratio of parity bits to systematic bits in the LDPC coding is determined by the desire to correct the bits having the lowest respective levels of confidence that they are correct when the Shannon limit is approached during AWGN reception conditions.
The LDPC coding prescribed in the DVB-T2 DTV broadcasting standard does not employ multilevel coding (MLC), a concept introduced by H. Imai and S. Hirakawa in their paper “A new multilevel coding method using error correcting codes” appearing on pages 371-377 in the May 1977 issue of IEEE Transactions on Information Theory, Vol. 23, No. 3. The key idea in MLC is that the bits of binary FEC coding are mapped to M-ary modulation symbol constellations for transmission. Decoding is expedited by FEC coding the bits mapped to different points in M-ary modulation symbol constellations independently of each other, which permits parallel independent decoding (PID).
A paper titled “Design of low-density parity-check codes for bandwidth efficient modulation” and authored by Jilei Hou, Paul H. Siegel, Laurence B. Milstein and Henry D. Pfister that appeared on pages 24-26 of the conference publication of IEEE ITW 2001 held Sep. 2-7, 2001 in Cairns, Australia is of interest. The authors reported they were able to achieve asymptotic performance very close to the capacity of an AWGN channel using multilevel coding employing irregular LDPC coding optimized for each code level. The Hou et alii ITW 2001 paper describes multilevel LDPC coding with different-rate LDPC codes for different levels, with the different-rate LDPC codes having codewords all of the same length. The ITW 2001 paper describes the mapping of the multilevel LDPC coding to 4PAM and 8PSK modulation symbol constellations. This paper and subsequent ones by other authors recognized that the use of Gray mapping and PID at each level separately with optimally chosen constituent codes offers performance approaching the Shannon limit on theoretical capacity of an AWGN channel.
LDPC coding of the different levels into which the information bit stream is split was proposed and described by I. B. Djordjevic and B. Vasic in their paper “Multilevel coding in M-ary DPSK/differential QAM high-speed optical transmission with direct detection” appearing on pages 420-428 of Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. An MLC scheme of data transmission using different constituent LDPC coding for each level was described by R. Y. S. Tee, O. Alamri, S. X. Ng and L. Hanzo in their paper “Block-Coded Sphere-Packing-Aided Multilevel Coding” appearing on pages 4173-4178 of IEEE International Conference on Communications, June 2007.
The above-cited references do not describe multilevel coding being applied to Gray-mapped quadrature amplitude modulation (QAM) of a plurality OFDM carrier waves. Uniform QAM provides a two-dimensional form of digital modulation in which the in-phase and quadrature-phase components of the complex amplitude modulation of each of the OFDM carrier waves can be independently coded and Gray mapped. Separate levels of MLC coding are provided for each of the bit positions in the labeling of points in the lattice defined by the different amplitudes and phases of the QAM symbol constellations. When Gray mapped appropriately, square QAM symbol constellations have an important property not known to have been fully previously exploited. The likelihood of corruption by random noise in each of the bits in the plural-bit labels of points in the square QAM symbol constellation that specify positioning in real-axis direction corresponds to the likelihood of corruption by random noise in one of the bits of the plural-bit labels of points in the QAM symbol constellation that specify positioning in imaginary-axis direction. The inventor found this significant because each such pair of bits that essentially are equally likely to be corrupted by random noise can be conveyed in the same level of multilevel LDPC coding. This halves the number of constituent codes in the multilevel LDPC coding and reduces the number of different coding algorithms transmitter requires for generating the constituent LDPC codewords in the multilevel LDPC coding. There is a corresponding reduction in the number of different decoding algorithms required for decoding the constituent LDPC codewords of the multilevel LDPC coding in receiver apparatus.
If the DTV transmitter selects systematic data bits to constituent codestreams of the MLC packet-by-packet, rather than bit-by-bit, the amounts of quite accurate timing needed in DTV transmitter and in DTV receiver apparatus are reduced. All the systematic data bits for each codeword of constituent coding are selected in original sequential order from a supply of all systematic data bits for all multilevel coding.